1. Field of the Invention
This invention relates to a recording/reproducing method of a magneto-optical recording medium, and a magneto-optical recording medium drive system utilizing such recording/reproducing method.
2. Prior Art
A magneto-optical recording medium is a recording medium wherein local heating of the magnetic thin film is achieved by means of a laser beam or the like to reduce coercive force, and wherein the direction of magnetization in this part is reversed with external magnetic field, or alternatively, the magnetization direction of initialization is maintained to thereby form a magnetic domain (record mark), and wherein the magnetization direction of the magnetic domain is read out by utilizing Kerr effect or Faraday effect.
In conventional magneto-optical recording media, density of the reproducible magnetic domains has been limited by the diameter of the laser beam spot used for the reproduction, and the magnetic domains of a diameter less than half of the laser beam spot diameter has been hardly reproducible.
Japanese Patent Application Kokai No. (JP-A) 8-7350 discloses an exemplary magneto-optical recording medium wherein reproduction of the magnetic domain with a diameter less than half of the laser beam spot diameter has been enabled. This magneto-optical recording medium has a recording film of triple magnetic layers, namely, first, second and third magnetic layers disposed on the substrate in this order, and the three magnetic layers are exchange-coupled. In the reproduction, the medium is irradiated with a laser beam from the side of the substrate, and reproduction magnetic field is applied from the side of the recording film. The recorded magnetic domain is retained in the third magnetic layer, and the magnetic domain is copied to the first and the second magnetic layers by laser beam irradiation. The thus copied magnetic domains enlarge in in-plane directions of the magnetic layers upon application of the read magnetic field. The copied, enlarged magnetic domain is read out as in the case of conventional magneto-optical recording media. When the reproduction of the copied, enlarged magnetic domain is completed, the copied magnetic domain is erased by applying erase magnetic field whose direction is opposite to the read magnetic field. By repeating such procedure, small magnetic domains which had been unreproducible could be reproduced. The merit of this process is not only the high reproduction resolution. Since the magnetic domains are actually enlarged, real amplification of the reproduced signal intensity is enabled. The magneto-optical recording medium of this type is hereinafter referred to as a magneto-optical recording medium of magnetic amplifying magneto-optical system (MAMMOS).
In this reproduction system, a modulated magnetic field comprising the reproduction magnetic field and the erase magnetic field should be applied in accordance with the density of the recorded magnetic domain. More illustratively, 1 cycle of the modulated magnetic field should be applied per 1 recorded magnetic domain (1 bit) for reproduction. In the recording in a magnetic field modulation mode, the recorded magnetic domain is formed by reversal in the direction of the recording magnetic field, and a modulated magnetic field (recording magnetic field) of 0.5 cycle at maximum is applied per 1 recorded magnetic domain (1 bit). As a consequence, in the medium of magnetic amplifying magneto-optical system which had been recorded in the magnetic field modulation mode, the frequency of the modulated magnetic field applied in the reproduction had to be twice the maximum frequency of the recording magnetic field. For example, in the example of the JP-A 8-7350, supra, the magnetic domain recorded at a disc linear velocity of 2.1 m/s and a modulation frequency of 2.5 MHz is reproduced by means of a magnetic field modulated at 5 MHz.
There is, however, a limit in terms of technology and cost in increasing the frequency of the magnetic field generating means, and the frequency of the modulated magnetic field used in the reproduction is determined by this limit. Therefore, the maximum frequency of the recording magnetic field is limited to 1/2 of the frequency limit of the magnetic field generating means, and increase in the data transmission rate upon recording is difficult in the medium of the magnetic amplifying magneto-optical system.